1. Field of the Invention
The present invention relates to the structure of a heat radiator for radiational cooling in a heat generating device, and more particularly to the structure of a heat radiator for radiational cooling which is suitable for use in a traveling-wave tube mounted on a satellite, etc.
2. Description of the Related Art
A traveling-wave tube is a device which accelerates electrons by a high voltage and converts a kinetic energy of electrons into an electromagnetic wave energy to amplify an electromagnetic wave.
FIG. 10 is a schematic cross-sectional view showing a traveling-wave tube. The traveling-wave tube is roughly made up of an electron gun section 7, a circuit section 8 and a collector section 9.
A high voltage is applied between an anode 12 and a cathode 10. Electrons 13 emitted from the cathode 10 are accelerated by the anode 12. Reference numeral 11 denotes a beam formation electrode, and reference numeral 4 denotes an insulating ceramic.
Electrons 13 having a large kinetic energy pass through a slow-wave circuit 16. A synchronous magnetic field for beam convergence is developed by a permanent magnet 14 for electron beam convergence and a magnetic pole 15. On the other hand, a signal inputted through an input window 17a is outputted from an output window 17b through the slow-wave circuit 16.
In the slow-wave circuit 16, the input signal from the input window 17a and the electrons 13 are interacted with each other in such a manner that a part of the kinetic energy of the electrons 13 is converted into the electromagnetic energy of the input signal. As a result, the kinetic energy of the electrons is reduced whereas the input signal is amplified. The amplified signal is outputted from the output window 17b.
The electrons 13 that have lost a part of kinetic energy are collected by a multi-stage collector electrode group 5 that constitutes a collector section 9. A first collector electrode 5a which is situated on the electron beam input side is applied with a relative high voltage, but the voltage is lowered toward the electron beam end side in such a manner that a fourth collector electrode 5d is applied with a voltage which is close to a voltage applied to the cathode 10.
With the above structure, electrons low in velocity are collected by the first collector electrode 5a whereas electrons high in velocity are collected by the fourth collector electrode 5d.
However, since the electrons 13 do not collide with the collector electrode group 5 at a velocity of 0, heat is always developed on the collector electrode group 5. The efficient radiation of the heat and the suppression of the collector temperature as low as possible are required for stable operation of the traveling-wave tube.
In particular, in the traveling-wave tube mounted on the satellite, heat must be radiated, as much as possible, directly toward space as much as possible, so that heat is not transmitted to the inside of the satellite. The effect of heat radiation due to radiation of the traveling-wave tube mounted on the satellite deeply depends on the structure of the heat radiator and the radiation coefficient .epsilon. of the surface of the heat radiator.
Also, since a plurality of tubular bulbs are disposed adjacent to each other on the satellite, a structure having a heat radiation directivity tends to become important in order to suppress heat interference of the respective tubular bulbs with each other as much as possible.
The prior art that takes the above circumstances into account will be described below.
FIGS. 11A, 11B and 12 show an example of the emissive heat radiator disclosed in Journal "Space", combined Nos. 11 and 12 of 1994, pp. 18 to 20.
FIGS. 11A and 11B are a front view and a side view of the emissive heat radiator, respectively, and FIG. 12 is a perspective view of the appearance of the emissive heat radiator. The emissive heat radiator is made up of a cylindrical section 18 and a plurality of radiating fins 19, where a collector is inserted into the cylindrical section 18.
A heat that flows into the cylindrical section 18 of the emissive heat radiator from a collector electrode is propagated to the radiating fins 19 due to heat conduction, and is then discharged to the exterior of the emissive heat radiator due to radiation from the radiating fins 19.
In this example, the radiating fins 19 are designed radially with respect to a center axis of the collector. Therefore, heat radiation have almost no directivity in both of radial and axial directions.
FIG. 13 shows an emissive heat radiator disclosed in U.S. Pat. No. 5,260,623, wherein two funnel-shaped sections 20, i.e., truncated cone-shaped projections and one annular disk-shaped section 26 are provided on a cylindrical section 3. A heat which flows into the cylindrical section 3 from a collector electrode inserted therein is propagated to the funnel-shaped sections 20 due to heat conduction, and is then radiated from the funnel-shaped sections 20. All surfaces of the sections 20 and 26, with the exception of the surface of the disk-shaped section 26 facing an arriving electron beam are characterized by high heat emission. In contrast, the surface of disk-shaped section 26 facing the arriving electron beam is designed as a surface with low heat emission.
The traveling-wave tubes on a satellite are arranged in such a manner that, as shown in FIG. 14, collector sections 22 (including an emissive heat radiator) are projected from the satellite 21 into a space and arranged adjacent to each other in a line. With the traveling-wave tubes thus arranged, a large number of traveling-wave tubes are mounted on the satellite, and heat generated from the collectors is radiated directly toward the space, to thereby make the cooling of the satellite efficient.
However, according to the prior arts shown in FIGS. 12 and 13, heat radiated laterally from each collector is absorbed by other collectors of the adjacent traveling-wave tubes, to thereby develop heat interference. As a result, the efficiency of radiational cooling is deteriorated.